1. Field of the Invention
The present invention relates to magnetic resonance image (MRI) technology, and particularly to a method for suppressing residual motion artifacts and a device for suppressing residual motion artifacts.
2. Description of the Prior Art
K-space is an important concept in MRI technology. K-space is an abstract space, and is also called Fourier Space. Magnetic resonance (MR) imaging data are arranged at specific positions in k-space according to different space frequencies, which are used to describe certain energy propagating in the form of waves in space, therefore, k-space can also be understood as a space filled with the original data of MR signals having space location encoding information. Each MR image has its corresponding K-space data, and by Fourier transform performed on the k-space data, the MR images can be reconstructed.
K-space can be two-dimensional, and it can also be three-dimensional. The abscissa Kx and the ordinate Ky in two-dimensional k-space are perpendicular to each other, and if Kx represents the phase encoding direction of the MR signals, then Ky represents the frequency encoding direction (also called readout direction) of the MR signals, as shown in FIG. 1. The Kx axis, Ky axis, Kz axis in a three-dimensional K-space are perpendicular to one another, and if Kx represents the phase encoding direction of the MR signals, Ky represents the frequency encoding direction, then Kz represents the level selection direction of the MR signals.
Referring to the two-dimensional k-space shown in FIG. 1, the sampling process for the MR signals is as follows: the direction and the field strength of the phase encoding gradient field of the MR signals are changed according to a preset step, the MR signals are sampled once for each time this change is made, and the sampled MR signals are filled into a corresponding line in K-space, the line is parallel to the readout direction and it is called a phase encoding line. In the figure, a0, a1 . . . an are shown in a phase encoding lines. As to a three-dimensional k-space, there is also a process similar to the above sampling process.
As mentioned above, the MR imaging data are actually arranged at specific positions in k-space according to different energy levels, and the energy in k-space follows an exponential decline from the middle to two sides, therefore, the energy in k-space is mainly distributed in a concentrated way in the middle of k-space and the two sides adjacent to the middle.
When an object being sampled is in a state of motion, what is sampled each time will not be a static object, but rather a number of objects on the trajectory of the object, that is to say, certain energy will always be sampled along the trajectory of this object, and such energy will leave some shadows on the MR images along the phase encoding direction. These shadows are called motion artifacts. It has been found in the clinical application of MRI technology that a human body's physiological movements, the movements of blood flow and so on will all lead to the production of motion artifacts during a imaging process, and which in turn cause the deterioration of the diagnostic quality by the magnetic resonance. There are various technical solutions for suppressing such movement in prior art, and two typical ones are introduced herein below.
The first technical solution: The physiological movements, such as by a heart, breath and so on, are synchronized by using various gate-control technologies so as to achieve the goal of filtering out the motion artifacts from the magnetic resonance images.
The second technical solution: The signals originated from the motion tissues are suppressed by using a space pre-saturating technology, so as suppress the signals, such as those from blood flow, brain liquid, etc.
Both of the above technical solutions assume that the physiological movements in a human body, the motion of blood flow, etc. are in accordance with some certain fixed rules, but in fact, the above assumption is not valid, because physiological movements in human bodies, motion of the blood flow, etc. usually are not completely regular. This leads to the case that there are always a small amount of residual motion artifacts left in the magnetic resonance images when using the above technical solutions.
As is apparent from the above discussion, after the motion artifacts are suppressed by using the prior art solutions, some residual motion artifacts will be left on the magnetic resonance image, therefore, using the prior art cannot suppress the motion artifacts very well.